The invention generally relates to reactive hot melt urethane adhesives. More particularly, the invention relates to a hot melt urethane adhesive including: an isocyanate compound; a polyester-polyol compound; a reactive tackifying resin including non-polar polyols; and a thermoplastic polymer.
Both hot melt adhesives and moisture cure, hot melt urethane adhesives are well known generic adhesive classes. Hot melt adhesives can be conveniently applied by extruding the adhesive composition at elevated temperatures directly onto a work piece for the purpose of forming a structural bond with another work piece as the temperature of the adhesive compositional mass cools. While hot melt adhesives have many adhesive preparation and workpiece production benefits, they have an effective temperature use range which is lower than its application temperature. In other words, the bonding mass can lose bond strength as the temperature of the work piece and the bond line increase.
In sharp contrast, moisture curable, hot melt urethane adhesives have little green strength in the their open state, limited peel and shear strength after set and require curing before these properties improve. After application of a urethane adhesive, the joined workpieces can require external mechanical support until the urethane adhesive can cure to a strong resilient, crosslinked bondline. Cured polyurethane bonds have high tensile strength and maintain their strength above their application temperature, 100-130xc2x0 C. Such adhesives have value where initial green strength is not important since substantial bond formation requires a period of curing time, which can range from hours to a number of days.
Clearly an adhesive displaying the high initial peel and shear strength of a hot melt which builds in strength over time is desirable. Early attempts to formulate such an adhesive have been made, notably attempts disclosed in U.S. Pat. Nos. 4,775,719 and 4,808,255, but none have been entirely successful. These early formulas set as a hot melt, but produced only modest increases in heat resistance when fully cured. This stems from the fact that only prepolymers made of non:polar polyols are compatible with APAO""s, EVA and other commonly used hot melt polymers. Likewise the ingredients, which are used to modify and formulate hot polymers, are too non-polar to mix with most polyester or polyether based urethanes. The most polar hot melt ingredients contain carboxylic acids or high level of hydroxyl groups, which are unstable with NCO groups at elevated temperatures.
Some of these early adhesives result in adhesive formulations having good green strength, but short open times, low heat resistance as cured, (i.e., low peel adhesion failure temperature (PAFT)), and limited adhesion to plastics. Accordingly, a substantial need exists in finding better hot melt adhesive formulations.
The inventors have developed compatible blends of components that form hot melt curing urethane adhesives with heat stability, green bond strength and fully cured bond strength that are surprisingly compatible in production and use. The reactive hot melt adhesive composition of this invention includes an isocyanate compound; a polyester-polyol compound; a reactive tackifying resin including non-polar polyols; and a thermoplastic polymer. The components of the adhesive composition cooperate in at least some embodiments to form a hot melt compatible adhesive composition that has substantial initial green strength, substantial cured bond strength, chemical and heat resistance when cured, extended pot life, long open times, high heat resistance, (i.e., high PAFT), and good adhesion to plastics.
In at least one embodiment, the invention includes a hot melt adhesive composition including: an isocyanate compound; a polyester-polyol; a reactive tackifying resin which includes a terpene-phenolic copolymer resin having a hydroxyl number of about 50 or less; and a thermoplastic polymer.
In at least one embodiment, the invention includes a method of increasing temperative resistance and open time in a hot melt adhesive composition, the method including incorporating into the adhesive an isocyanate compound; a polyester-polyol, and a reactive tackifying resin including a non-polar polyol having a hydroxyl number of about 50 or less.
The inventors have found useful hot melt adhesive compositions that include: an isocyanate compound; a polyester-polyol compound; a reactive tackifying resin including non-polar polyols; and a thermoplastic polymer.
The term xe2x80x9cisocyanate compoundxe2x80x9d in the context of this invention indicates a typically monomeric small molecule having 2 or more NCO groups. Isocyanate compounds useful for forming the compositions of the invention include organic, aliphatic and aromatic isocyanate compounds having an isocyanate functionality of about 2 or more. The isocyanate compounds can also contain other substituents which do not substantially adversely affect the viscosity of the isocyanate terminated compositions, the adhesive properties of the bond line or the reactivity of the NCO groups during the formation of the composition. The isocyanate compound can also include mixtures of both aromatic and aliphatic isocyanates and isocyanate compounds having both aliphatic and aromatic character.
Typical aromatic isocyanate compounds include diphenylmethane diisocyanate compounds (MDI) including its isomers, carbodiimide modified MDI, diphenylmethane-4,4xe2x80x2-diisocyanate, diphenylmethane-2,2xe2x80x2-diisocyanate, diphenyl-methane-2,4xe2x80x2-diisocyanate, oligomeric phenyl methylene isocyanates; toluene diisocyanate compounds (TDI) including isomers thereof, tetramethylxylene diisocyanate (TMXD), isomers of naphthylene diisocyanate, isomers of triphenylmethane triisocyanate, and mixtures thereof. Aliphatic di, tri, and polyisocyanates are also useful including for example isophorone diisocyanate, hydrogenated aromatic diisocyanates, aliphatic polyisocyanates, cycloaliphatic polyisocyanates, and others.
The polyester-polyol compounds can be produced by reacting polyols with a polyfunctional carboxylic acid compound.
Suitable polyols that can be used as reactants to form the polyester-polyols have a molecular weight in excess of 250, more typically in excess of 500, most preferably in the molecular weight range of about 500-10,000. Typical polyols include monomeric diols, triols, etc. and polymeric diols, triols comprising a chain of repeating methylene units providing a major contribution to the molecular weight of the material. A typical polymeric polyol includes either a linear or branched chain of the repeating units which is terminated with hydroxyl groups and for the sake of simplicity these hydroxyl groups will normally be the active hydrogen containing substituents in the polyol structure reacting with the capping isocyanate compounds.
Preferred polyols include monomeric polyols having 2 or 3, hydroxyl groups per molecule. Representative, but exclusive examples include ethylene glycol, propylene glycol, glycerine, trimethylol propane, 1,2,6-hexane triol, butene diol, 1,4-butanediol, 1,3-propanediol, neopentyl glycols.
Some preferred polyols include hexane diol, higher polyalkylene diols, cyclic polyols containing 1 to 3 -hydroxyl groups and a cycloaliphatic structure. Examples of such preferred diols include 1,4-dihydroxymethyl cyclohexane, hydrogenated bisphenol-A, hexane diol, dodecane diol, or other higher molecular weight diols. Especially preferred polyols include neopentyl glycols.
Under certain conditions release of vaporized isocyanate compositions can occur during manufacture or application of the adhesive of the invention. In order to reduce isocyanate volatility, a small amount of a low moleoular weight polyol can be added, with the polyester-polyol, that can react with free isocyanate compounds.
The polyester-polyols can be produced by reacting the above discussed polyols with a polyfunctional carboxylic acid compound, preferably a higher (C8 or higher) paraffinic diacid. Such polyfunctional acids are primarily monomeric compositions having two or more carboxylic acid groups. Representative examples of such acids include dodecane dioic acid, azelaic acid, sebacic acid, 1,18-octadecane dioic acid, dimer acid prepared from a mono-, di- or triunsaturated fatty acid, acid wax, acid anhydride grafted wax, or other suitable polycarboxylic acid reacting compound.
A preferred polyester-polyol for use in this invention includes the reaction product of dimer acid and either a 1,4-dihydroxyalkyl cyclohexane or hydrogenated bisphenol-A, dimer acid being a composition made by reacting active olefinic groups in a C12-24 unsaturated fatty acid resulting in crosslinking between two fatty acid molecules.
In the reactive hot melt of this invention the isocyanate compound reacts with the terminal hydroxyl group to form isocyanate terminated compositions having free NCO groups for reaction with moisture or other reactive hydrogen containing compounds. In at least some embodiments, only the polyester-polyol is reacted with the isocyanate compounds and the tackifying resins to produce the hot melt adhesive.
In at least some other embodiments, however, a mixture of the above discussed polyester-polyol compounds with polyether-polyol compounds may be used. In these embodiments, an appropriate amount of the polyester-polyol compound used is replaced with an appropriate amount of a suitable polyether-polyol compound to form a mixture of polyester-polyol compounds and polyether-polyol compounds. This mixture of polyester-polyol /polyether-polyol compounds is then used to react with the isocyanate compounds and the tackifying resins to produce the hot melt adhesive. The ratio of polyester-polyol to polyether-polyol in these mixtures is preferably in the range of about 10:1 to about 1:5, and more preferably about 5:1 to about 1:2.
Suitable polyether-polyol compounds for use in such a mixture have two or more hydroxyl groups in the molecule. Preferably, the polyether-polyol compounds are diols or triols of a molecular weight resulting in an appropriate viscosity. Typically, suitable polyether-polyol compounds have a molecular weight of less than about 20,000, or typically less than about 10,000, preferably in the range of about 1,000 to about 10,000, and most preferably for reasons of reactivity and viscosity control, in the range of about 2,000 to about 4,000. Examples of preferred polyether-polyol compounds include polyoxyallylene compounds having at least two hydroxyl groups in the molecule, and polyalkylene ether glycol compounds. Specific examples of suitable polyether-polyol compounds include polyethylene glycols, polypropylene glycols, tetramethylene glycols, polybutylene glycols, and mixtures thereof.
The adhesives of the invention also contain a reactive tackifying resin. The inventors have found that in at least some embodiments, when used with the urethane components discussed above, such reactive tackifying resin can increase the PAFT values of the urethane adhesive, and extend the open time of the polymers without compromising heat stability. The reactive tackifying resins also increase the viscosity of liquid components.
The reactive tackifying resins are preferably non-polar polyols, which are solid at ambient temperatures, and preferably have the following characteristics: functionality between 1 and 2.5; hydroxy numbers in the range of about 50 or less and preferably in the range of about 50 to about 30; soften points above 0xc2x0 C. and preferably in the range of 50xc2x0 C. to 130xc2x0 C.; acid numbers below 1; and are non-polar. (i.e., able to form compatible and stable blends when co-reacted with the above discussed pre-polymers.)
Examples of suitable reactive tackifying resins include: terpene-phenols, low acid number reaction products of rosin acid and diepoxies, and hydroxyl modified rosin esters. Especially preferred reactive tackifying resins include terpene-phenols with hydroxy numbers in the range of about 20 and about 50.
Below is a table containing some commercially available useful reactive tackifying resins:
The reactive hot melt urethane adhesive composition of the invention contains a compatible thermoplastic polymer which cooperates with the other adhesive components to provide initial green strength and cured strength to the inventive compositions. Preferably, the thermoplastic polymer composition is matched in polarity with the urethane components and with the tackifying resins. The preferred thermoplastic copolymer component of this invention includes an ethylene vinyl monomer polymer such as an ethylene vinyl acetate copolymer, an ethylene acrylic monomer copolymer, or ethylene acrylic carbon-monoxide copolymer.
The polyethylene-vinyl monomer composition can be a film-forming thermoplastic polymer compatible with the adhesive components of the invention. Preferably the vinyl monomer of the ethylene vinyl monomer composition comprises an acrylate monomer or a vinyl ester monomer of a carboxylic acid compound. Acrylate monomers that can be used in the film-forming polymer of the invention include acrylic acid, methacrylic acid, acrylamide, methacrylamide, methylacrylate, ethyl acrylate, methylmethacrylate, 2-ethylhexylacrylate, 2-ethylhexylmethacrylate, methoxyethylmethacrylate, methoxyethylacrylate, ethylene n-butyl acrylate, and others. A preferred acrylate monomer is ethylene n-butylacrylate which includes about 30-50% butylacrylate. Acrylate monomers are well known in the art and are selected for copolymerization with ethylene based on the polarity of the monomer. Vinyl esters of carboxylic acids include such monomers as vinyl acetate, vinyl butyrate, and others.
The preferred isocyanate capped components of this invention are formulated to have a polarity compatible with ethylene-vinyl acetate polymers (EVA) having about 10-60 wt-% vinyl acetate and a melt index of about 0.2 to 1000.The most preferred EVA comprises a copolymer with about 18 to 50 wt-% vinyl acetate and a melt index of about 0.2 to 500. The compositions of polymers using other second monomer systems should be formulated to obtain a polarity approximating the polarity of these EVA compositions.
The adhesives of the invention may optionally contain a non-reactive tackifying resin in combination with the isocyanate, the polyester-polyol, the reactive tackifying resin and the thermoplastic polymer. The non-reactive tackifying resins useful in adhesives of the invention comprise aromatic, aliphatic or aliphatic aromatic tackifying resins.
Aromatic resins useful in forming the adhesive compositions of this invention can be prepared from any monomer containing an aromatic moiety and a polymerizable ethylenically unsaturated group. Typical examples of aromatic monomers include styrenic monomers such as styrene, alpha-methyl styrene, vinyl toluene, methoxy styrene, tertiary butyl styrene, chlorostyrene, etc., indene monomers including indene, methylindene and others. Aliphatic aromatic tackifying resins can be made by polymerizing feed streams containing a mixture of one or more aliphatic monomers in combination with one or more aromatic monomers. Typically aromatic aliphatic resins can be formed from the aromatic monomers above in mixture with generally unsaturated petroleum feedstocks which contain C4 +monomers.
The mixed aromatic-aliphatic resins contain aromatic monomers recited above polymerized with aliphatic monomers, preferably with the C4-6 monomers recited below.
Both natural and synthetic terpene tackifying resins and synthetic aliphatic hydrocarbon resins can be used in the compositions of the invention as non-reactive tackifying agents.
Typical monomers making up aliphatic hydrocarbon resins include 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, 2-methyl-1,3-butadiene, 2-methyl-2-butene, cyclopentadiene, dicyclopentadiene, and others.
In at least some embodiments, the preferred non-reactive tackifying resin used depends upon the type of thermoplastic polymer used. For example, in at least some embodiments the preferred non-reactive tackifying resin may depend upon the wt-% VA content of the EVA if EVA is used as the thermoplastic polymer. For example, for EVA resins having a wt-% of VA greater than 28-33%, aromatic or aliphatic-aromatic resins having a ring and ball softening point of 70xc2x0-120xc2x0 C. are preferred. For EVA resins having a wt-% of VA less than 28% aliphatic or aliphatic-aromatic resins having a ring and ball softening point of 70-120xc2x0 C. are preferred. Preferred resins are generally C9 and hydrogenated C9 resins. In some embodiments pure monomer resins -methyl styrene can be used, but it may shorten open times in final materials. Additionally, pure monomer resins may have some compatibility problems at 33% VA EVA and below.
The ratio of the tackifying resins (both reactive and non-reactive) in relation to the thermoplastic polymer can effect the open time and initial peel resistance of the adhesive before the adhesive moisture cures. At any urethane component level, increasing the tackifying resin to thermoplastic polymer ratio will lengthen the open time. For open times beyond 1 minute, the tackifying resin to thermoplastic polymer ratio is preferably greater than 1, and is more preferably between 2 and 3. Additionally, resins which have good initial peel resistance have tackifying resin to thermoplastic polymer ratios that yield tan deltas of 0.50 to less than 1.00 upon cooling from the melt as measured by dynamic mechanical analysis (DMA).
The compositions of the invention can contain other compatible polymers, fillers, pigments, dyes, catalysts, inhibitors, antioxidants, UV absorbers, waxes and other conventional additives. The fillers may be in the form of particles or fibers of compositions including ceramics, glass, silica, quartz, mica, treated clay, titanium dioxide, boron nitrides, graphite, carbon black, glass fibers, asbestos fibers, metal powders, etc., the amount of filler depending on particular properties of the composite desired.
In somewhat greater detail, the reactive hot melt urethane adhesives of at least some embodiments of the invention typically comprise an effective bonding amount of a thermoplastic polymer base to provide initial green strength, an effective amount of a polyurethane composition including the reaction product of (i) a polyester polyol compound, and (ii) an isocyanate compound to provide final cured strength, and chemical and heat resistance; a reactive tackifying resin to provide for increased open times, and increased PAFT values; and a compatible tackifying resin. The reactive hot melt urethane adhesive composition is typically blended under anhydrous inert gas mixing conditions to form a uniform adhesive admixture which is then typically packaged in a variety of sizes ranging from about 25 g. to 250 kilogram mixtures. In use the compositions can be placed in hot melt application equipment under an inert gas blanket, heated to melt temperature for extrusion.
The reactive hot melt urethane adhesive compositions of the invention can be cured in the bond line using a variety of mechanisms. The curing reaction occurs between a compound having an available active hydrogen atom and the NCO groups of the polyurethane precursor. A variety of reactive compounds having free active hydrogens are known in the art including water, hydrogen sulfide, polyols, ammonia, and other active compounds. Such curing reactions can be conducted by relying on the presence of moisture in the atmosphere.
Typically the adhesive compositions of this invention can be prepared using the proportions of components found in the following Table 1:
In at least some embodiments, the weight ratio of the combined weight of the polyester-polyol and isocyanate components in relation to the reactive tackifying resin is about 1:1 to about 4:1, preferably about 2:1 to about 3:1.
While the choice of component, order of addition, and addition rate can be left to the skilled adhesives chemist, generally the reactive hot melt urethane adhesives of this invention can be made by preparing the polyester-polyol compound (or mixture of polyester-polyol and polyether polyol) and blending the polyester-polyol (or mixture of polyester-polyol and polyether polyol) with the reactive tackifying resin, the thermoplastic polymer and other optional ingredients such as the non-reactive tackifying agent, if necessary, into a suitable reaction vessel at elevated temperatures, typically in the range of 100xc2x0-250xc2x0 F. (38xc2x0-122xc2x0 C.). The isocyanate compound is then introduced into the reaction vessel at elevated reaction temperatures, typically in the range of 100xc2x0-300xc2x0 F. (38xc2x0-149xc2x0 C.). Moisture is typically excluded from reaction using dry chemicals and conducting the reaction under vacuum or the presence of an anhydrous gas blanket.
The polyester polyol compound (or the mixture of polyester-polyol/polyether-polyol) is reacted with the isocyanate compound in the reactive vessel at ratios that typically depend on the hydroxy and isocyanate functionality of the reactants. Typically the compounds are reacted at ratios which result in a reaction between isocyanate groups and hydroxy groups leaving essentially no residual hydroxy and minimal isocyanate functionality, typically less than 10 wt-%. Typically the reaction between the polyester polyol compound (or mixture of polyester-polyol and polyether polyol) and the isocyanate compound is conducted at an OH:NCO ratio of about 0.75 to 0.16:1 in order to obtain an NCO concentration in the final adhesive of about 1 to 5%. Typically the resulting urethane material is titrated to measure residual concentration of isocyanate using ASTM D-2572-80 xe2x80x9cStandard method for isocyanate group and urethane materials or prepolymersxe2x80x9d. If the prepolymer composition has acceptable viscosity and isocyanate content, it can be packaged in suitable moisture resistant containers or immediately blended with the balance of the components to form the finished adhesive of the invention.
The adhesives of the invention can be formed into a single package combining the polyester polyol compound (or mixture of polyester-polyol and polyether polyol), isocyanate compound, the reactive tackifying resin, the thermoplastic polymer, and any optional components, such as the non-reactive tackifying agent. Typically the combination can be made with standard batch or continuous industrial reaction or blending equipment having suitable agitation, temperature control, vacuum and inert atmosphere. Again, in the formation of the finished adhesive composition, blending of the individual components can be left to the skilled adhesives formulator. The components are mixed at a rate such that the ingredients are smoothly combined.
The hot melt adhesives of the invention can be used in accordance with uses generally known in the art for hot melt adhesives. For example, the adhesive can be used as adhesives in roll coating applications, such as panel assembly operations, or as sealants, such as sealants around windows, and a broad variety of other such applications. The adhesive can be conveniently applied directly onto a substrate at elevated temperature for the purpose of forming a structural bond with another substrate as the temperature of the adhesive compositional mass cools. Before curing, the adhesive generally exhibits sufficient green strength such that external mechanical support is not needed while the adhesive cures. Once the adhesive cures, it binds the two substrates firmly together.